A hologram is a three-dimensional image of an object that has been recorded and is later reproducible. The recordation of a hologram requires the use of a monochromatic coherent light source that is split into two components. One of the components of the coherent light source is used to illuminate an object whose photographic image is to be recorded. The light waves that are reflected from the object contain all possible optical information regarding the object and subsequently impinge upon a photographic plate which is exposed by the light waves reflected by the object. The second component of the coherent light is incident upon the same photographic plate and has a wave front of known and reproducible form, known as the reference beam. The wave front of the reference beam and the light waves reflected from the object interfere to create a diffraction pattern uniquely related to the object, which is the hologram.
In classical holography, the image of the object is reproduced by viewing in a beam of monochomatic coherent light similar to that of the reference light. The coherent light is diffracted by the hologram identical in form and direction to that originally emanating from the object, and an observer viewing this diffracted wave front sees a three-dimensional stereoscopic image of the original object.
The reconstruction of the holographic image as just described requires illumination by a laser to create the necessary diffraction pattern. The laser beam must have the appropriate orientation and cross-sectional area. However, there exist techniques for producing information limited holograms that are viewable under white light. An example of this is described in U.S. Pat. No. 3,633,989 issued to Benton entitled "Method for Making Reduced Bandwith Holograms," and commonly referred to as a rainbow hologram. U.S. Pat. No. 3,633,989 describes two processes for the making of orthoscopic holographic images. One of these involves the preparation of a hologram, which, upon illumination with a monochromatic coherent light beam conjugate to the reference beam used in recording, yields a parallax limited real pseudoscopic image. This real image is then holographically recorded to form a second hologram. The second hologram, when illuminated by a beam conjugate to the reference beam used in recording it, yields a real orthoscopic image. The second approach described in U.S. Pat. No. 3,633,989 comprises holographically recording an image of an object formed by a lens provided with an elongated horizontal slit aperture with the aid of a diverging reference beam. Illumination of the hologram so formed with a converging beam conjugate to the source produces a real image of the slit aperture through which a real orthoscopic image of the scene or object can be viewed.
Other setups and methods of producing rainbow holograms have been proposed A common element of all these methods to create the rainbow holograms of the prior art, however, is the use of a straight, horizontal slit aperture or masking means having a large aspect ratio to limit the amount of information in the recorded hologram. The straight, horizontal slit rainbow hologram eliminates vertical parallax without sacrificing a three-dimensional appearance, since depth perception depends essentially upon the horizontal parallax Since the illuminating light is white light, a series of strips of different colors are formed The color of the image changes from red to violet as the observer lowers his or her eye position, which is the reason this type of hologram is referred to as a rainbow hologram.
The image of a hologram produced using a straight, horizontal slit aperture may therefore be seen only from a certain "window" in front of the hologram. The window of visibility is dictated by the length of the slit relative to the size of the object. Thus, if a hologram is mounted on a wall as a picture, the hologram may be viewed from directly in front and from either side up to a certain distance. If one moves further to the side, the image abruptly disappears. If one moves up or down the color of the hologram varies through and beyond the visible spectrum. Thus, eventually one sees a red image and then nothing as the spectrum moves into the infrared. Similarly, at the other extreme, the image disappears as one goes beyond the visible blues.
The use of a straight, horizontal slit aperture in the formation of a rainbow hologram therefore forms a window of visibility having no vertical parallax and a horizontal viewing angle that is ordinarily dictated by the length of the slit, the viewing angle usually being much less than 180.degree.. There are instances, then, where the hologram may be positioned in such a manner that this particular viewing window is not optimal An example would be a two-dimensional rainbow hologram which is positioned to lie flat on a table or inlaid into its surface such that the viewer cannot lean over to view the hologram directly. Under such circumstances, the viewing zone provided by the straight, horizontal slit aperture proves inadequate.
There are holograms of the prior art which can provide up to a 360.degree. view of an object. See, for example, U.S. Pat. Nos. 3,784,276 issued to Wuerker et al. and 4,339,168 issued to Haines. These holograms are typically recorded by use of a right circular cylinder of film which surrounds the object. For reproduction purposes, the arrangement is the same except that the object is removed. Though cylindrical holograms provide a 360.degree. viewing angle, such holograms and similar deviations therefrom (such as a conical shape) require a less convenient three-dimensional format. Generally, the two-dimensional format is preferred because of a greater number of applications available for use. Rainbow holography is capable of reducing three-dimensional imagery to a two-dimensional format and subsequently reconstructing the three-dimensional imagery, though, as described above, current rainbow holography methods have restricted viewing angles.